This book is about time. I would like to have named it Time, the Forgotten Dimension, although such a title might surprise some readers. Is not time incorporated from the start in dynamics, in the study of motion? Is not time the very point of concern of the special theory of relativity? This is certainly true. However, in the dynamical description, be it classical or quantum, time enters only in a quite restricted way, in the sense that these equations are invariant with respect to time inversion, t —> — t. Although a specific type of interaction, the so-called superweak interaction, seems to violate this time symmetry, the violation plays no role in the problems that are the subject of this book.